In general, conventional dual band filters having two pass bands can be classified into two types in terms of configuration.
One type is a filter composed of dual band resonators that have an appearance of one integral unit, resonate at two frequencies and are coupled to the input/output ports and further dual band resonators coupled thereto, such as the filter shown in FIG. 14 (see the non-patent literature 1, for example). For this filter, the structure and the dimensions of the coupling parts of the dual band resonators disposed at the opposite ends and coupled to the input/output line have to be determined to achieve a desired center frequency and a desired bandwidth for each of the two bands.
The other type is a filter composed of a plurality of transmission lines having different impedances and different lengths connected at the respective ends to each other, such as the filter shown in FIG. 15 (see the non-patent literature 2, for example). For this filter, the characteristics of a dual band filter are achieved by determining the characteristic impedance and the length of each transmission line based on the equivalent circuit theory using lumped elements.    Non-patent literature 1: S. Sun, L. Zhu, “Novel Design of Microstrip Bandpass Filters with a Controllable Dual-Passband Response: Description and Implementation,” IEICE Trans. Electron., vol. E89-C, no. 2, pp. 197-202, February 2006    Non-patent literature 2: X. Guan, Z. Ma, P. Cai, Y. Kobayashi, T. Anada, and G. Hagiwara, “Synthesizing Microstrip Dual-Band Bandpass Filters Using Frequency Transformation and Circuit Conversion Technique”, IEICE Trans. Electron., vol. E89-C, no. 4, pp. 495-502, April 2006